Conventionally, the Land Mobile Radio (LMR) system in the United States has extremely low spectrum efficiency because frequencies are allocated to respective channels at extremely wide channel spacing such as 25 kHz or 30 kHz. To improve this spectrum efficiency, the Federal Communications Commission (FCC) of the United States provided in the FCC rule that, from 1997, the channel spacing be changed to 12.5 kHz that was half the conventional spectrum efficiency. Consequently, the LMR system was operated at the channel spacing of 25 kHz or 12.5 kHz. This change of the channel spacing is explained in the FCC Rule (Title 47 Code of Federal Regulations PART 90: Private Land Mobile Radio Services)).
To further improve the spectrum efficiency, in 2005, the FCC rule for changing the channel spacing to 6.25 kHz, which is half the present spectrum efficiency, will be enforced. Therefore, in accordance with the enforcement of the FCC rule in 2005, it is necessary to develop an LMR system that can be operated even at the channel spacing of 6.25 kHz.
The FCC rule to be enforced in 2005 provides, as conditions for the LMR system, that the LMR system have spectrum efficiency capable of operating one sound channel per 6.25 kHz band and have a transfer rate equal to or higher than 4800 bps per 6.25 kHz band. It is possible to allow the LMR system to operate one sound channel per 6.25 kHz band by adopting the FDMA (Frequency Division Multiple Access) system in the 6.25 kHz band or the 4-slot TDMA (Time Division Multiple Access) system in the 25 kHz band. In recent years, since it is a general practice to transmit character data and the like in addition to sound data, it is desirable that the LRM system to be developed for the enforcement of the FCC rule in 2005 is capable of communicating not only sound data but also character data and the like.
First, the FCC rule that sets the standard of the present LMR system applicable to the channel spacing of 12.5 kHz will be explained. The FCC rule (Title 47 Part 90.210: Emission masks) prescribes emission masks corresponding to respective bands. A mask D having characteristics shown in Table 1 below and FIG. 1 is stipulated for the 12.5 kHz band.
TABLE 1Displacement FrequencyRangeAttenuation (dB)fd < 5.625 kHz05.625 kHz < fd < 12.5 kHz7.27 (fd − 2.88)fd > 12.5 kHz70 or 50 + 10 log10(P)whichever the lesser ATT
In the table, fd is a displacement frequency range from a center frequency and is represented by a unit kHz. P is a transmission power and is represented by a unit W.
Most of the present LMR systems applicable to the channel spacing of 12.5 kHz transmit an analog sound signal after subjecting the analog sound signal to Frequency Modulation (FM modulation) (analog FM modulation). A sound band and a maximum frequency shift of the LMR systems are as shown in Table 2 below.
TABLE 2Modulation SystemFM ModulationSound Band0.3 kHz to 3 kHzMaximum Frequency Shift±2.5 kHz Shift
The modulation system (APCO P25 Phase 1 modulation system, hereinafter “P25-P1 modulation system”), which was examined as the Project 25 in the APCO (Association of Public Safety Communications Officials) and, then, enacted as the standard (TIA102) by the TIA (Telecommunications Industry Association) is also used as a modulation system of the LMR system applicable to the channel spacing of 12.5 kHz. This modulation system is a system for transmitting a digital signal of a base band according to four-level FSK modulation. A transmission rate, a symbol rate, a base band filter, and a nominal frequency shift of the modulation system are as shown in Table 3 below.
TABLE 3Transfer Rate9600 bpsSymbol Rate4800 symbol/sBase Band FilterTransmission: A filter obtainedby combining a filter having aRaised Cosine characteristic withα = 0.2 and a shaping filterReception: An integrate and dumpfilterModulation SystemFour-Level FSK ModulationNominal FrequencyShifts of +3 = +1.8 kHz, +1 = +0.6 kHz,Shift−1 = −0.6 kHz, and −3 = −1.8 kHzfor respective four symbol levels(±3, ±1)
In measuring an emission spectrum at the time when the analog FM modulation system is used, the FCC rule provides, as a measurement condition, that a modulation frequency be set to 2.5 kHz and that the emission spectrum be measured by being modulated at a level increased by 16 dB from a modulation signal level at which 50% of a maximum frequency shift is obtained. A waveform of the emission spectrum set to satisfy this condition and the emission mask (mask D) are shown in FIG. 2. As shown in FIG. 2, it is provided that the emission mask be substantially at the same level as a line of a high-order component of 2.5 kHz.
A waveform of an emission spectrum measured using pseudo-random data as a modulation signal in the P25-P1 modulation system and the emission mask (mask D) are shown in FIG. 3. Since the pseudo-random data is used as the modulation signal, as shown in FIG. 3, the emission spectrum measured has a uniformly distributed spectrum shape and conforms to the mask D.
A waveform of an emission spectrum measured using data with symbols +3 and −3, that is, data with shifts +1.8 kHz and −1.8 kHz rather than the pseudo-random data and the emission mask (mask D) are shown in FIG. 4. Since a symbol rate is 4800 symbol/s and a rectangular wave is shaped to a sine wave by a base band filter, the emission spectrum measured is equivalent to a spectrum subjected to FM modulation with a frequency shift of a certain value by a sine wave of 2.4 kHz. As shown in FIG. 4, the emission spectrum measured has a peak at a frequency integer times as high as 2.4 kHz and third-order and fourth-order components of the emission spectrum slightly deviate from the mask D.
The FCC rule stipulates a mask E having a characteristic shown in Table 4 below and FIG. 5 for the 6.25 kHz band.
TABLE 4Displacement FrequencyRangeAttenuation (dB)fd < 3.0 kHz03.0 kHz < fd < 4.6 kHz65 or 30 + 16.67(fd − 3) or55 + 10 log10(P) whicheverthe lesser ATTfd > 4.6 kHz65 or 55 + 10 log10(P)whichever the lesser ATT
In order to adapt an emission spectrum to the mask E, the analog modulation FM modulation system is applied as the modulation system in one case and the P25-P1 modulation system is applied as the modulation system in another case. These cases will be examined.
In the past, when the channel spacing was revised from 25 kHz to 12.5 kHz, an emission spectrum could be adapted to the emission mask (mask D) stipulated by the FCC rule by changing a frequency shift in the analog modulation FM modulation system from the 5 kHz shift to the 2.5 kHz shift. A waveform of an emission spectrum measured by changing a frequency shift from the 2.5 kHz shift to the 1.25 kHz shift following this example (at a modulation frequency of 2.5 kHz) and the emission mask (mask E) are shown in FIG. 6. As shown in FIG. 6, obviously, the emission spectrum measured does not conform to the emission mask.
A waveform in the case in which a transmission rate and a frequency shift were set to half as large as those at a channel spacing of 12.5 kHz and pseudo-random data was used as a modulation signal in the P25-P1 modulation system and the emission mask (mask E) are shown in FIG. 7. A waveform of an emission spectrum in the case in which data with a symbol alternately taking ±3 was used as a modulation signal in the P25-P1 modulation system and the emission mask (mask E) are shown in FIG. 8.
Since the emission spectrum shown in FIG. 7 is uniformly distributed, the emission spectrum seemingly conforms to the emission mask. However, actually, as shown in FIG. 8, the emission spectrum does not conform to the emission mask.
Simply by halving the parameters such as a transmission rate and a frequency shift of the modulation system currently applied to the LMR system applicable to the channel spacing of 12.5 kHz in this way, it is impossible to adapt the emission spectrum to the emission mask (mask E).
A case in which another modulation system is used for the LMR system to adapt an emission spectrum to an emission mask (mask E) will be explained. For example, a case in which a modulation system of the APCO P25 Phase 2 standard (hereinafter, “P25-P2 modulation system”) is applied to the LMR system will be examined. This P25-P2 modulation system is a system in which a modulation system on a transmission side is changed to the π/4QPSK modulation system while keeping a data format of the P25-P1 modulation system as it is. A transmission rate, a symbol rate, a base band filter, and a phase shift of this modulation system are as shown in Table 5 below.
TABLE 5Transmission Rate9600 bpsSymbol Rate4800 symbol/sBase Band FilterTransmission: A filter having aRaised Cosine characteristic withα = 0.2Reception: An integrate and dumpfilterModulation Systemπ/4QPSK modulationPhase ShiftShifts of +3 = +¾p, +1 = +¼p,−1 = −¼p, and −3 = −¾p forrespective four symbol levels(±3, ±1)
A waveform of an emission spectrum measured when pseudo-random data was modulated by the P25-P2 modulation system and the emission mask (mask E) are shown in FIG. 9. Since the P25-P2 modulation system is based on the n/4QPSK modulation system, as shown in FIG. 9, the emission spectrum measured has a characteristic of steeply attenuating out of a band and conforms to the emission mask (mask E) despite the fact that the transmission rate is 9600 bps.
However, since the n/4QPSK modulation system is a linear modulation system, problems described below occur.
It is impossible to use a nonlinear power amplifier used in the present LMR system. In order to use a linear power amplifier in the LMR system, since an additional circuit such as a linearizer is required, a space and cost for the LMR system increase. Since the linear power amplifier has lower efficiency and a larger consumed current compared with the nonlinear power amplifier, heat generation in a radio apparatus constituting the LMR system causes a problem. Moreover, in a portable LMR system, since the portable LMR system is driven by a battery, an operation time decreases.
At this moment, a linear power amplifier having an output power that is the same as that of the conventional nonlinear power amplifier and having efficiency equivalent to that of the conventional nonlinear power amplifier has not been developed. In addition, it is extremely difficult to make a mounting space and cost of a nonlinear power amplifier equivalent to those of the non-linear power amplifier. Therefore, it is not realistic to apply the linear modulation system represented by the P25-P2 modulation system to the LMR system applicable to the channel spacing of 6.25 kHz.
The invention has been devised to solve the problems and it is an object of the invention to provide a modulating apparatus, a mobile communication system, a modulating method, and a communication method that can conform to the FCC rule to be enforced in 2005 without using a linear power amplifier.